Density element and method of manufacture thereof

ABSTRACT

A density element (12) for use in ruminal delivery devices (10) which is manufactured by partial sintering in such a manner as to fragment upon contact with the many parts in rendering machinery without damage to the blades. The density element (12) has density of at least about 1.5 gm/cm3 and a transverse rupture strength greater than about 3000 psi no greater than about 30,000 psi. The part is sintered under conditions which do not permit full weld bond strength to be obtained and may thereafter be heat treated or impregnated with an inert hydrophobic material to increase corrosion resistance.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of pending U.S. patentapplication Ser. No. 07/768,285, filed Oct. 3, 1991, now abandoned whichwas a continuation-in-part of application Ser. No. 07/335,028, filedApr. 7, 1989, now abandoned, which was a division of U.S. patentapplication Ser. No. 07/591,923, filed Oct. 2, 1990, now abandoned.

TECHNICAL FIELD

This invention relates to ruminal drug delivery devices and particularlyto density elements for such devices and methods for their manufacture.

BACKGROUND OF THE INVENTION

Ruminant animals, including cattle, sheep, giraffe, deer, goats, bisonand camels, and more particularly the domesticated species comprise animportant group of animals that require periodic administration ofmedicines, nutrients and other biologically active agents (which arehereinafter referred to in their broadest sense as "drugs") for thetreatment and alleviation of various conditions and for better health.

Ruminants have a complex three or four compartment stomach, with therumen being the largest compartment. The rumen, including the reticulum(hereafter referred to as the "rumen") serves as an important organ forlocating dispensing device for delivering medicines and nutrients tosuch animals over extended periods of time.

There are numerous ruminal delivery devices known in the art which arecapable of prolongedly releasing drugs. Typically, these devices areswallowed by the ruminant or otherwise mechanically introduced into therumen by means of a bolus gun for example, and remain therein for a longperiod of time without being regurgitated or otherwise eliminated.Typical devices are those disclosed in U.S. Pat. Nos. 4,505,711,4,578,263, 4,595,553, 4,612,186, 4,623,345, 4,642,230, 4,643,393 and4,675,179 incorporated herein by reference.

In order to insure: that these devices remain in the rumen for aprolonged period of time a density element is often incorporated intothe device. Typically, the density element is manufactured from amaterial such as iron or steel, iron oxide, magnesium, zinc, cobaltoxide, copper oxide or mixtures thereof, metal shot or parts which maybe coated with iron oxide, zinc, magnesium alloy, copper oxide, mixturesof cobalt oxide and iron powder and unsintered, compacted metal powders,and the like. Such density elements typically have sufficient density tobring the overall density of the delivery device to a level greater thanthe density of ruminal fluid (approximately 1 gm/cm³) and typically toan overall density of at least 2 gm/cm³.

In animals such as cattle raised for slaughter the density element willremain in the carcass after slaughter. The rumen and ruminal contents ofanimals still containing ruminal delivery devices, including theirdensity elements, are typically processed by rendering plants. Renderingplants comprise a highly automated and continuous operation and thoughsuch machinery is typically equipped with magnetic retrieval systems,these systems are not always effective for removing the densityelements. As a result, the density elements have caused extensive andcostly damage to grinder blades, guillotines, rollers and otherequipment.

In addition, it has been found that density elements made from materialssuch as iron, magnesium or zinc which corrode in water or ruminal fluidand generate gases which interfere with the proper operation of fluidactivated delivery devices such as those shown in U.S. Pat. Nos.4,595,553, 4,612,186 and 4,675,174, for example.

DISCLOSURE OF THE INVENTION

It is an object of this invention to provide structurally coherentdensity elements for ruminal delivery devices having a densitysufficient to maintain the delivery device in the rumen of a livinganimal and also reproducibly fragment into harmless particles withoutdamage to machinery when the density element contacts the rollers andblades in the cutting and grinding equipment of a rendering plant. Asused herein, the term "structurally coherent" refers to density elementswhich are generally monolithic in nature and are physically broken intosmaller particles on contact with rendering blades; as distinguishedfrom density elements formed of individual non-coherent elements, suchas metal shot, contained in a rupturable container which are dispersedupon contact with rendering blades.

Another object of this invention is to provide structurally coherentdensity elements having a transverse rupture strength less than or equalto that of bovine or ovine bone.

A further object of this invention is to provide structurally coherentdensity elements for use in ruminal delivery devices having a densitysufficient to maintain the device in the rumen for a long period of timeand a transverse rupture strength less than or equal to bone.

It is another object of this invention to provide fluid actuated ruminaldelivery devices having corrosion resistant, structurally coherentdensity elements possessing a transverse rupture strength less thanbovine or ovine bone.

It is another object of this invention to provide structurally coherentdensity elements having a transverse rupture strength greater than"green" strength and no greater than about 30,000 psi.

It is another object of this invention to provide structurally coherentdensity elements having a transverse rupture strength in the range ofabout 6,000 psi-30,000 psi.

It is another object of this invention to provide processes formanufacturing structurally coherent density elements that are resistantto corrosion in water or ruminal fluid.

It is another object of this invention to provide processes formanufacturing structurally coherent density elements for use in ruminaldelivery devices that will reproducibly disintegrate into small harmlessparticles upon contact with blades and rollers used in renderingmachinery.

According to this invention a metal powder is compressed and thereaftersintered at a temperature below the standard sintering temperature forthe metal at which weld bond strength is achieved (hereinafter,"partially sintered"). The partial sintering may be accomplished ineither a reducing, inert or oxidizing atmosphere to produce densityelements having various properties as will be explained in detail below.If partially sintered in a reducing or inert atmosphere, the densityelement may thereafter be heat treated in an oxidizing atmosphere toincrease corrosion resistance. The partially sintered parts may also beimpregnated with an inert, preferably hydrophobic, material such assilicon oil, mineral oil or wax to further increase corrosionresistance. A non-alloyable filler material may also be mixed with themetal powder prior to compression to reduce the inter-particle bondstrength of the partially sintered, structurally coherent densityelement.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings, which are not drawn to scale, but rather are set forth toillustrate the various embodiments of the invention and wherein likereference numerals designate like parts, are as follows:

FIG. 1 is a partial cross sectional view of a ruminal delivery devicehaving one embodiment of the structurally coherent density element ofthis invention; and

FIG. 2 is a partial cross-sectional view of a ruminal delivery devicehaving another embodiment of the structurally coherent density elementof this invention.

DESCRIPTION OF THE INVENTION INCLUDING BEST MODES

This invention will be described with respect to ruminal deliverydevices of the type shown in the Figures, but it is not limited to thespecific devices disclosed. The ruminal delivery device designsillustrated herein are merely exemplary of devices known to the art asgenerally described above and the density elements of this invention canbe manufactured in any configuration and be adapted to fit in a ruminaldelivery device of any type or configuration.

FIG. 1 shows a fluid activated device 10 of the type described in thepatents noted above having a structurally coherent density element 12 atthe bottom of the device. The device would also be designed with a wall14 which surrounds an internal capsule wall 16 and defines an internallumen 18, which is partially shown in FIG. 1. The agent to be deliveredcan be dispersed throughout a composition 20, which is delivered througha passageway 22 by pressure exerted upon said composition by afluid-expandable member 24.

The density element 12 is flat bottomed so as to fit the contour ofdevice 10. However it can have any shape desired and if the ruminalbolus device has a rounded bottom, the density element can likewise beshaped to conform to the curve.

This invention also contemplates positioning the density element nearthe external passageway as is shown in the device 26 of FIG. 2. With thedensity element 28 so positioned, the passageway 30 extends through thedensity element 28 to the agent containing composition 20 containedwithin device 26. For purposes of illustration only, device 26 differsfrom device 10 by having only a single wall 14 and having a roundedbottom 32.

The structurally coherent density elements of this invention arecharacterized by having: a) a density sufficient to maintain thedelivery device within the rumen of the animal to which it isadministered; and b) a transverse rupture strength that will allow thestructurally coherent density element to fragment into harmlessparticles or pieces without damage to rollers, cutting blades or othermoving equipment that may contact the density element in the renderingprocess.

Density elements according to this invention should have a density of atleast about 1.5-8 gm/cm³ or higher and preferably the density is withinthe range of about 2.2 to 7.6 gm/cm³. For ruminal bolus devices whichare administered to cattle or sheep, it is preferred to use a densityelement such that there is a resulting overall density of the deliverydevice of at least about 3 g/ml.

The structurally coherent density elements of this invention will alsohave a transverse rupture strength no greater than the maximum strengthfor which blades in rendering equipment are designed to be capable ofrupturing or disintegrating without damage to the equipment. Thisstrength is that of ovine or bovine bone which is approximately 30,000psi (210 kg/cm²). The structurally coherent density elements of thisinvention should also have a transverse rupture strength greater thanabout 3000 psi (210 kg/cm²) which is the maximum strength normallyobtained by compaction of the particles in making "green" parts, asdiscussed below and preferably above about 6000 psi.

Transverse rupture strength of a material is determined by standard ASTMtest, ASTM B(378)-7, in which samples of a specified configuration aresubjected to a standardized test. Because the density elements of thisinvention have a different configuration than that utilized in thestandard tests, the transverse rupture strength of the elements of thisinvention may be determined by measuring the transverse rupture strengthof standard shaped test samples manufactured under the same conditionsas the density elements of this invention.

The transverse rupture strength of parts having non-standardconfigurations, such as the cylindrical parts of the Figures, may alsobe determined indirectly from another parameter, radial crush force. Ina radial crush test the density element is crushed to yield between twoparallel plates and the force at yield measured is measured. Becauseradial crush force is a geometry dependent property, an initialcorrelation between radial crush force and transverse rupture strengthmust be made by tests on samples of the particular geometry having knowntransverse rupture strengths. Once the correlation is establishedtransverse rupture strength can be determined from radial crush tests ofthe structurally coherent density elements themselves.

The structurally coherent density element of this invention can bemanufactured from any dense, preferably metallic material, which wouldnot react with the ruminal fluid in a manner that would interfere withits functioning as a density element. Iron, because of its density,cost, chemical and biological properties and attraction to magneticretrieval systems, is preferred according to this invention.

The structurally coherent density elements of this invention arecomprised of a partially sintered agglomeration of dense particles, thatwill reproducibly rupture and disintegrate into component particles,smaller agglomerates or powder upon impact with grinding blades or otherenergetic components encountered in rendering plants without damagingthe equipment.

Sintering is a process of heating small metallic particles toagglomerate them into bulk materials by establishing metallurgical bondsbetween the particles. The bonds are produced by the formation of aliquid phase between the particles or by solid diffusion between theparticles. In typical sintering processes of the prior art, the metallicparticles are compressed into the desired configuration to form acompacted, unsintered (hereafter, "green"), relatively fragile partwhich is thereafter heated for a time and at a temperature sufficient topermit weld bonds to form between the particles. As a result, typicalsintering processes produce a metal product which exhibits strengthproperties approaching those of metals subjected to conventionalmetallurgical processes which involve melting of the metallic material.The partial sintering process of this invention, however, is conductedunder conditions which prevent the formation of full weld bonds andthereby can provide a product having a density similar to that obtainedfrom a typical sintering process but a much lower transverse rupturestrength than would be obtained by typical sintering procedures.

The size of the unsintered high density sinterable powder used willaffect the density and transverse rupture strength of the finishedproduct and the preferred particle size is 100%<100 mesh and 85%<325mesh. To further reduce the transverse rupture strength of the endproduct, the sinterable powder can optionally be combined with silicapowder or another suitable high density, nonmetallic or non-alloyablemetallic filler material that will interfere with the formation of weldbonds between the particles to be sintered. The filler material wouldhave a particle size comparable to that of the sinterable powder and ispreferably present in amounts of from 0-50% by volume. A small amount ofa lubricant may also be added to the mixture to facilitate uniformcompression in the formation of the green part as is known in thesintering art. Suitable lubricants include waxes and oils and maytypically be present in amounts of about 0-5% by wt.

The addition of a filler will decrease the transverse rupture strengthof the structurally coherent density element and, since typical fillersare less dense than the sinterable material, will also decrease thedensity. The particle size of the filler material will also have aneffect on the strength and density of the finished item and can bevaried to obtain the desired combination of density and transverserupture strength. Generally, larger particle sizes of the material to besintered and the filler will produce lower density end items and largerfiller particles and smaller sinterable particles will produce lowertransverse rupture strengths of the finished product.

The sinterable powder/filler particle mixture is compressed into thedesired configuration and to approximately the desired density in asuitable die. The compression force should be at least sufficient toprovide green strength adequate to permit handling of the part in itsgreen state. Green strength within the range of about 1000-3000 psi andpreferably about 1700-1800 psi are suitable. Typically the compressionforce required to achieve adequate green strength is within the range ofabout 10-40 tons/in² and preferably about 30 tons/in². Green strength isdetermined by standard ASTM test B(312)-7. Correlations between radialcrush force and green strength can also be made in the same manner asdescribed above.

Compression is followed by partial sintering at a temperature below thestandard sintering temperature used to achieve weld bond strength forthe sinterable material forming the density element. For iron, thepreferred partial sintering temperatures according to this invention arein the overall range of about 1100°-1600° F. and preferably at about1200°-1300° F. for about 1-2 hours. The appropriate temperatures forother materials will also be less than the conventional sinteringtemperatures for such materials and can be readily determined by workersskilled in the art.

The partial sintering may be performed in either reducing, inert oroxidizing atmospheres which are selected to produce the characteristicsdesired for the particular density element.

If the partial sintering is performed in a reducing or inert atmospherethe part may thereafter be heat treated in an oxidizing atmosphere. Theheat treatment can be performed at temperatures ranging from about500°-1500° F. to produce an oxidized finish which improves corrosionresistance. The partially sintered part may also be impregnated with aninert, preferably hydrophobic, material such as mineral oil, corn oil,microcrystalline wax or the like to further improve corrosionresistance.

The characteristics obtained from various combinations of partialsintering and heat treatments of iron powder are summarized at Table 1.

                  TABLE 1                                                         ______________________________________                                        Partial                                                                              Heat                                                                   Sintering                                                                            Treat-                                                                 Atmos- ment     Trans-   Corro-                                               phere  @ 1000-  verse    sion                                                 @ 1200-                                                                              1300° F.                                                                        Rupture  Resis-                                               1500° F.                                                                      in air   Strength tance Comments                                       ______________________________________                                        Oxidizing                                                                            None     Lowest   Poor  External surface of                            (air)                          structure oxidizes                                                            closing pore                                                                  structure. Lubricant                                                          vaporizes and emerges                                                         under high vapor                                                              pressure. Conversely,                                                         air does not easily                                                           diffuse back in                                                               through tight pore                                                            structure and                                                                 interior not                                                                  oxidized. Lowest                                                              inter-particle                                                                strength.                                      Inert  None     Low      Poor  Lubricant removed.                             (Nitro-                        Exterior and interior                          gen)                           not oxidized. Low                                                             inter-particle bond                                                           strength.                                      Inert  Yes      Medium   Good  Lubricant removed.                             (Nitro-         Low            Exterior and Interior                          gen)                           not oxidized. Higher                                                          inter-particle bond                                                           strength because less                                                         oxide present during                                                          partial sintering.                             Reducing                                                                             None     Medium   Poor  Lubricant removed.                             (endo                          Exterior and interior                          gas)                           not oxidized. Higher                                                          inter-particle bond                                                           strength than in                                                              inert gas because of                                                          elimination of oxide                                                          during partial                                                                sintering.                                     Reducing                                                                             Yes      Highest  Good  Lubricant removed.                             (endo                          interior and exterior                          gas)                           oxidized. Highest                                                             inter-particle bond                                                           strength results from                                                         subsequent heat                                                               treatment.                                     ______________________________________                                    

DESCRIPTION OF BEST MODES cl EXAMPLE I

Hollow cylindrical samples configured as shown in FIG. 2, O.D. 0.91,I.D. 0.20 in., length 1.33 in. were formed by compressing 99% wt ironpowder (100%<100 mesh, 85%<325 mesh) and 1% petroleum based waxlubricant such as Accra Wax in a suitable die at 30 tons/in² to achievea green density of 6.83 gm/cm³ and a green strength of 1770 psi. Thesamples were then partially sintered in an oxidizing atmosphere (air) at1300°-1500° F. for 1-2 hours. The parts so formed had an oxidizedcorrosion resistant exterior coating which blocked the pore structureand made subsequent oxidation of the interior impractical. The parts hada crush strength in the range of 1800-2300 pounds which was equivalent,for this configuration, to a transverse rupture strength of about6,000-7,700 psi. Some of these parts fragmented during normal handlingin the subsequent manufacturing process in which delivery devices werefabricated from these elements which indicates that these parts approachthe lowest practical strength according to this invention. Structurallycoherent density elements manufactured according to this example willfragment without damaging rollers or blades in rendering machinery andare suitable for use in ruminal delivery devices that do not utilizefluid activated dispensing means because measurable hydrogen gasevolution, as a result of corrosion of the unoxidized interior of thedensity element, will occur when immersed in water or ruminal fluid.

EXAMPLE II

A green density element formed as in Example I was partially sintered ina reducing atmosphere composed of "endo" gas made by cracking naturalgas with air over a catalyst at 2050° F. for 30 minutes which wasthereafter cooled to approximately 1500° F., forming a mixture of H₂,CO, CO₂ and N₂. The parts were partially sintered at about 1400° to1500° F. for from one to two hours. The wax lubricant was removed duringthe sintering operation leaving a porous unoxidized structure having acrush strength of approximately 3500 pounds equivalent to a transverserupture strength of approximately 12,500 psi. Samples so manufacturedwere subjected to a fragmentation test by impact with a stainless steeltool blade having a 1 millimeter thick edge at a velocity of 2 metersper second. All samples fragmented without damage to the blade which wascomparable to blades used in rendering machinery. The samples evolvedsignificant amounts of hydrogen gas upon immersion in a manner similarto the density elements of Example I.

EXAMPLE III

A green density element formed as in Example 1 was partially sintered atabout 1300°-1500° F. in nitrogen for one to two hours and thereafterheat treated at 1050°-1350° F. in air. The lubricant was removed duringthe furnace treatment in nitrogen to produce an open pore structure andboth the interior and exterior surfaces of the part were oxidized duringthe subsequent furnace treatment in air. The part exhibited a crushforce of approximately 4000 pounds which is equivalent to approximately18,500 psi transverse rupture strength. The parts are fragmentable uponimpact with blades in rendering machinery and will not evolve measurablequantities of gases that would interfere with the operation of fluidactuated drug delivery devices when immersed either in water or ruminalfluid.

EXAMPLE IV

Green density elements formed as in Example 1 were partially sintered at1300°-1500° F. for 1 to 2 hours in "endo" gas and thereafter blackenedat about 1000°-1300° F. in forced flowing air. The parts possessed acrush force of approximately 6,000 pounds corresponding to a transverserupture strength of about 20,000 psi. The lubricant was removed duringthe partial sintering operation and both the interior and exterior ofthe part were oxidized. Oxide originally present in the green part wasremoved during the sintering operation resulting in a slightly strongersintered product than obtained according to Example III. The parts arefragmentable upon contact with the rollers and blades in renderingmachinery and were oxidized both interior and exterior. When used as thedensity element in fluid actuated delivery devices they will not evolvemeasurable quantity of gasses that would interfere with the operation ofthe device when exposed to either water or a ruminal fluid.

EXAMPLE V

In an effort to improve the corrosion resistance of density elementsformed according to examples I and II, the porous elements wereimpregnated with a hydrocarbon wax (Multi-wax 180-M) at ambientpressures, positive pressures of 30 psi and in vacuum/pressure at 30 cmHg/80 psig according to the following procedures:

A. Ambient pressure impregnation

1. Heat density elements and wax separately in a forced air oven to 120°C.

2. Combine wax and density elements for 1 hour.

3. Remove density elements from the 120° C. and immediately directstream of air, water or steam at the tops of the density elements toblow away the wax hanging up at the tops of the elements.

4. Allow the density elements to cool to room temperature and place(standing upright) on four thicknesses of paper toweling.

5. Place density elements on paper toweling in 120° C. for 30 minutes.

6. Remove density elements while still on the paper toweling and allowto cool. The excess wax aggregate at the bottom of the density elementskirt will have been absorbed into the paper toweling.

B. Pressure Impregnation

1. Heat wax in pressure vessel to 120° C.

2. Heat density elements separately at 120° C.

3. Immerse heated density element upright in wax and seal pressurevessel. Bring vessel pressure up to 30 psi with nitrogen.

4. Place vessel in oven at 120° C. for 4 hours.

5. Remove vessel from oven and release pressure slowly.

6. Remove density elements from vessel and blow off excess top surfacewax with air, water or steam.

7. Allow to cool at room temperature.

8. Place on paper toweling and put into 120° C. for 30 minutes. Cooldensity elements to room temperature.

C. Vacuum Impregnation

1. Heat wax in stainless steel beaker to 120° C.

2. Heat density elements at 120° C.

3. Immerse density elements upright in wax and immediately transfer to120° C. vacuum oven holding at 30 centimeters of mercury for 4 hours.

4. Relieve vacuum.

5. Blow off excess top surface wax with air, water or steam.

6. Allow to cool to room temperature.

7. Place on paper toweling and put into 120° C. oven for 30 minutes.

8. Cool elements to room temperature.

D. Vacuum--Pressure Impregnation

1. Heat wax in a stainless steel vacuum/pressure rated, jacketed tank to120° C., leaving sufficient head space in tank to accommodate densityelements in a wire mesh basket to be later lowered into the molten wax.

2. Place density elements into basket suspended from tank lid byoperable lift/lower mechanism (typically hydraulic or air cylinder orscrew) sealed against pressure vacuum. Attach lid--basket--densifiersassembly to the top of the tank, thus closing the tank with densifiersinside.

3. Produce 30 cm Hg vacuum within tank and hold while densifiers reach120° C.

4. Activate lift lower mechanism to immerse hot (120° C.) densityelements into molten (120° C.) wax.

5. Reduce vacuum and raise pressure to 80 psi.

6. Hold for 4 hours or longer

7. Reduce pressure to ambient while keeping tank closed.

8. Activate lift/lower mechanism to raise baskets out of molten wax andhold them in the head space above tank to drain excess wax at 120° C.

9. Transfer baskets to another container for exposure to air, steam, orhot water to remove remaining excess wax.

10. Allow the densifiers to cool.

In all cases, the density elements produced according to Example IIabsorbed wax significantly faster than those produced by Example I.Pressure impregnation either alone or with vacuum was far superior inincreasing the amount of wax absorbed than either the ambient or vacuumonly impregnation techniques. The pressure impregnated samplesmanufactured according to Example II exhibited greater corrosionresistance than the unimpregnated samples.

Certain of the techniques employed in process D would be applicable tothe commercial manufacture of impregnated density elements according toprocesses A-C. Placing multiple elements in a movable basket within apressure vessel containing the wax, heating them simultaneously to 120°C. and then lowering the basket into the molten wax for impregnationmakes handling of multiple units simpler. After impregnation, raisingthe elements from the bath and allowing them to drain at 120° C.eliminates the steps of absorbing the excess wax on toweling anddisposing thereof.

EXAMPLE VI

The procedures of Example V are applied to density elements producedaccording to Examples III and IV. The products so obtained will exhibita combination of strength and corrosion resistance which makes them thepreferred embodiments for use in fluid actuated ruminal delivery devicesaccording to this invention.

EXAMPLE VII

The density elements can also be impregnated at ambient temperatures andpressures if a liquid impregnating material is used. Since an ambienttemperature and pressure impregnation has obvious processing advantages,the preferred impregnating composition consists of 84.5% by weight cornoil, 10% ethyl alcohol, 5% lecithin and 0.5% butylated hydroxytoluenemixed to homogeneity in a low shear mixer. The preferred density elementis a non-blackened element formed from 99% by weight of iron powder,sieve analyses (U.S. Std.) 60-0%, 100-10.3%, 32.5-69.8) pan-19.9% and 1%wax lubricant compressed to 6.7 g/cm³ and partially sintered for about30 minutes at 1400-1500 degrees F. in an endo gas or nitrogenatmosphere.

The resulting product was 0.91" O.D., 1.654" long with a 0.20" diametercentral bore, tapering externally toward the bore at one end andinternally toward the bore at the other end. The product had a crushstrength of less than 16,000 pounds of force at yield when crushedbetween flat plates which was equivalent to a tranverse rupture strengthless than 30,000 psi. Elements of this size should be impregnated withfrom 0.2-0.4 g of the impregnating solution.

The density elements were immersed in the impregnating solution atambient conditions for approximately 2.5 minutes which is adequate toprovide impregnation weights in the desired range.

This invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be affected within the spirit and scopeof this invention which is limited only by the following claims wherein:

We claim:
 1. A structurally coherent density element for use in aruminal delivery device said element comprising a partially sinteredagglomeration of high density particles having:a) a density of at least1.5 gm/cm³ and; b) a transverse rupture strength in the range of 6000psi to 30,000 psi such that said element fragments upon contact with theblades of rendering machinery without damage to the blades.
 2. Thedensity element of claim 1 wherein the density of the element is withinthe range of 2.2-7.6 gm/cm³.
 3. The density element of claim 1 whereinsaid element comprises partially sintered iron.
 4. The density elementof claim 3 wherein the surfaces of said density element which areexposed to ruminal fluid are oxidized.
 5. The density element of claim1, 2, 3, or 4 impregnated with an inert hydrophobic material.
 6. Aruminal drug delivery device having a density greater than b 1.0 gm/cm³comprising, in combination:a) a dose of a drug to be delivered to therumen of a ruminant animal; and b) a density element for maintainingsaid delivery device in the rumen of said animal, said density elementcomprising a structurally coherent, partially sintered, agglomeration ofhigh density particles having a density of at least 1.5 gm/cm² and atransverse rupture strength in the range of 6000 psi to 30,000 psi suchthat said density element fragments upon contact with moving cuttingblades without damage to said blades.
 7. The delivery device of claim 6wherein said density element has a density of from about 2.2-7.6 gm/cm³.8. The delivery device of claim 6 wherein said density element comprisespartially sintered iron.
 9. The delivery device of claim 7 wherein saiddensity element comprises partially sintered iron.
 10. The deliverydevice of claim 8 wherein the surface of said density element which areexposed to ruminal fluid are oxidized.
 11. The delivery device of claim6, 7, 8, or 10 wherein said density element is impregnated with an inerthydrophobic material.
 12. The delivery device of claim 6, 7, 8 or 10further comprising:c) fluid actuated means for driving said dosage ofdrug from the devices; and d) a density element that is resistant tocorrosion by ruminal fluid.
 13. The delivery device of claim 10 furthercomprising fluid actuated means for delivering said dosage from thedevice.
 14. The delivery device of claim 11 further comprising fluidactuated means for delivering said dosage from the device.